In the medical field, magnetic resonance imaging (MRI) is a commonly used non-invasive technique to diagnose the medical condition of a patient. MRI has the ability to distinguish healthy and diseased tissue, fat and muscle, and between adjacent structures within the body which other imaging modalities cannot demonstrate. MRI utilizes safe radio waves and a magnetic field to generate the images processed by a computer. Typically, the patient is placed within a large homogeneous magnetic field and is subjected to a set of gradient fields and radio frequency (RF) fields. The various fields are accurately controlled to cause nuclei within a selected slice of the patient to precess about an axis and to emit RF signals. These signals are then used to reconstruct an image of the slice. By varying the gradient fields, images of the patient at different slices may be captured. The separate slices can then be combined to form a complete scan of the patient.
Generally, with respect to the use of MRI scanners, video systems are employed for both (a) patient comfort and (b) functional imaging applications. With respect to patient comfort, the concern is directed to anxious or claustrophobic patients who resist entering the tunnel of the MRI scanner. The capability to adequately display visual information for viewing is an important factor for relief for the anxious or claustrophobic patient. The second use of video systems in MRI scanners is directed to functional imaging applications. In some instances, the diagnostic procedure performed with the MRI is used to evaluate a patient's response to specific visual stimuli. The operator sends a series of images to a screen which is seen by the patient during the MRI procedure and the patient's responses are included in the MRI report.
A problem with introducing conventional video signals into an MRI device is that very small magnetic fields generated by another device can destroy the images generated by the MRI device. Conversely, the strong fields generated by the MRI device may prevent the normal operation of certain devices, such as a cathode ray tube (CRT) or liquid display panel (LCD), within the vicinity of the MRI device. Therefore, any type of system used to present video signals to the patient must not generate any stray magnetic fields in the vicinity of the MRI device and should be shielded from the magnetic fields generated by the MRI device.
Another problem is that the MRI device is based on the use of radio frequencies that may disrupt signal modulation. For these reasons, the video signal must be in a form that is not affected by the radio frequency and transmitted by a system that is not easily magnetized.
The most common method for presentation of visual stimuli inside the MR scanner is to generate an image outside the magnetic field of the MR machine and have a mirror or prism for reflecting the image to the patient. For instance, viewing systems as described in U.S. Pat. No. 5,076,275 to Bechor et al., U.S. Pat. No. 6,774,929 to Kopp and an MRI video system disclosed in a Nuclear Associates brochure all reflect images generated from a video source located away from the patient into the eyes of the patient. The projection is achieved within the magnetic environment by employing an MRI-compatible LCD screen, or by using a video projector and a translucent screen. The screen is positioned in the proximity of the MR scanner. The projector or LCD screen is positioned either inside or outside the MR room. The video information is viewed by the patient with the aid of adjustable light reflecting mirrors or through a prism. The utility of this method of visual activation is limited by the position of the patient within the scanner tunnel. Further, the level of ambient light in the MRI magnet room will affect the quality of the image that the patient sees on the screen. A high level of ambient light will cause the screen image to be washed out. Also, the time required to adjust the light reflecting mirrors with respect to the screen is determined by the position of the patient inside the scanner tunnel. For functional magnetic resonance imaging, it is ideal to cover the entire patient field-of-view with the MRI screen or display.
The effectiveness of this method of visual activation is further reduced by an open field of view (e.g., the screen is outside of the tunnel) which enables the patient to be aware of her surroundings. Therefore, the patient may find it difficult to focus on the video images and may therefore find it difficult to completely relax. This may be especially true for systems which reflect the video images from behind the MRI device to the patient. With this type of system, the patient may be distracted by items which are adjacent to the display screen or by people working behind the patient. Thus, the possibility of being distracted by the external surroundings in addition to the interior of the tunnel further limits the usefulness of this technique for the reduction of anxiety and claustrophobia in patients. It would therefore be desirable to have the patient focus on the video images during the MRI procedure so that the patient is able to relax.
An attempt to address this problem is found in U.S. Pat. No. 4,901,141 which utilizes a fibre optic taper positioned within the bore of an MRI apparatus. In order to isolate the video system from the fields generated by the MRI device and to prevent any magnetic fields from affecting the MRI device, this system pipes in video images to the patient while the patient is within the MRI device. A CRT produced image is delivered to the fibre optic taper through a coherent image guide. The fibre optic taper expands the end of the image guide so as to provide a larger viewing surface for the patient. The problem with the fibre optic taper is that it is stationary and the patient must be positioned in a fixed location so as to be able to see the end of the optic taper. Further, to prevent distortion the patient must be located directly beneath the isocenter of the taper. Thus, the disclosure does not address different size patients, patient positioning, or near and far sighted patients. For instance, a tall person may lay with their head partially outside the bore during diagnostics of the lower body whereas a child may be well encapsulated by the bore, neither of which could properly see a fixed fibre optic taper. In addition; the use of a fixed taper will interfere with auxiliary coils, such as head and c-spine coils, that require close proximate to the body. Current construction of head and c-spine coils is such that the visual field as needed for viewing a fixed positioned fibre taper is either obscured or completely blocked if the fibre taper is utilized.
Another prior art device is disclosed in U.S. Pat. No. 5,414,459 directed to a pair of glasses worn by the patient. The glasses receive the video picture by fibre optic guide.
In both theses devices the installation is permanent with a fibre optic connection between the shielded MRI room and a remote location housing the operating elements of the system. The connection requires the shielding which surrounds the MRI room to be breeched and that penetration must be adequately protected.
Current MRI fibre optic systems that position the LCD screen within the scanner room (but outside the bore of the MRI scanner) are extremely useful and provide a definite advance in the art. Notwithstanding, certain features of this design could be improved. In particular, the length of the fibre optic bundle employed to carry the video images from the LCD screen to the eyepiece for viewing by the patient is of concern. As with all transmission systems, a portion of the transmitted parameter is lost during transmission and the longer the transmission path, the greater the loss. For long fibre optic bundles, it is known that the loss of as much as forty percent (40%) of the transmitted video image can occur. This loss affects the resolution and brightness of the transmitted video image. Therefore, the resolution and brightness of the transmitted video image is limited by the length of the fibre optic bundle. Additionally, the longer the fibre optic bundle, the more cumbersome it is to carry the bundle and associated fibre optic equipment into and out of the MRI scanner tunnel.
A fibre optic bundle is comprised of a plurality of optical fibres. When an optical fibre is interrupted, the pixels of light of the transmitted image carried by the interrupted fibre are blocked. This situation results in dead pixels, e.g., black spots that appear on the video display. As the length of the fibre optic bundle is increased, the probability that individual fibres will be broken increases. Further, as the fibre optic bundle is bent and manipulated over a period of time, the number of broken fibres increases. An increasing number of broken fibres results in a greater number of black spots appearing on the video display. Eventually, the transmitted image becomes inadequate and distorted. Thus, long fibre optic bundles are not cost effective.
During an MRI examination, the patient is positioned upon an examination table which can be moved into and out of the MRI scanner tunnel. When lying upon the examination table within the scanner tunnel, the patient's head is positioned within a head coil. The head coil is arranged to surround the patient's head and to provide MRI images thereof. An advanced design of MRI scanner head coils minimizes the distance between the patient's eyes and the top of the head coil. The limited distance between the patient's head and the head coil would be inadequate to accommodate the goggles employed by known MRI fibre optic systems that (a) position the image from the LCD display within the scanner tunnel or (b) employ a reflecting mirror over the patient's eyes.
The advance of the functional imaging field requires implementation of visual activation paradigms that are becoming more sophisticated. During functional imaging, the best results are achieved when the visual stimulus is controlled which is inconsistent with an open field of view. Further, this method of visual activation does not include the ability to generate three-dimensional (3D) images for patient viewing since the image is projected onto a single screen. The inability to create a condition is which the eye and brain perceive a 3D effect prevents virtual reality from being achieved.
Further, the development of new and smaller head coils limits the distance between the patient's head and the head coil, putting restraints on the size of the goggles to be used within the head coil. Together with the introduction of MR machines with higher field strength both in the clinical and research field, the shielding of the MR goggles to avoid generation of any stray magnetic fields or disruption of signal modulation by radio frequency is becoming increasingly important.
The use of functional imaging in clinical work also requires devices that are fast and easy to set up and operate in a tight clinical schedule. Easy positioning of the device and effective eye correction features are crucial elements to achieve a satisfactory clinical workflow.
Thus, there is a need in the art for an improvement in video systems for use with MRI scanners which provide high resolution video images with a three-dimensional effect, shortens the transmission paths that the video image must travel, eliminates the problems associated with fibre optic bundles, is sized to fit within the limited space of modern head coil designs, is sufficiently shielded to avoid image artefacts and can be mounted and operated within the MRI magnetic field.